The present invention relates to an engine idling control apparatus which is capable of precisely judging an engine idling state from an output signal of a throttle sensor.
During an idling operation of an engine, an engine speed is set to a predetermined value by regulating an intake air quantity flowing through a bypass for bypassing a throttle valve in dependency on a control of an idle speed control valve (ISCV). Ignition timings are set to a preset crank angle.
Conventionally, the engine idling state has been mechanically judged by an idling switch in dependency on operation of the throttle valve. If the switch is turned on, it is unconditionally judged that the throttle valve is fully closed, i.e., in the idling state, and the engine idling operation starts.
In practice, however, since it is necessary to turn on the idle switch definitely when the throttle valve is fully closed, the idle switch is arranged to turn on at a predetermined opening degree before the throttle valve is fully closed.
As the result., if an acceleration pedal is depressed to start a vehicle, although the engine speed rises, the idling control does not terminate until the throttle valve is opened over the predetermined opening degree. Furthermore, if the vehicle is decelerating, the idle switch is turned on and idling control starts in an incomplete state even though the throttle valve still opens.
In the case of a conventional engine, since an area change ratio of the throttle bore between the predetermined opening degree at which the idle switch turns on or off and a fully closed state of the throttle valve is small, that is, a changing range of intake air quantity is small, the start, the acceleration and the deceleration performance are not influenced largely by the operation of the idling switch mentioned above.
However, a latest engine has a throttle bore with a large diameter so as to obtain a high performance and a high output power. Even a small change in an opening degree of the throttle valve will therefore result in a large change in the intake air quantity. Accordingly, when idling control is performed or ended in response to a signal output from the idle switch, the engine outputs an insufficient power. As a result, not only the start, the acceleration and the deceleration performance are degraded, but also where the exhaust gas emission and fuel consumption are deteriorated. Furthermore, controllability of idling operation is also more difficult.
In order to deal with .the above problem, it can be considered that the idle switch may be set near a fully closed position of the throttle valve so as to ensure proper ON/OFF operation. However, the distance between the switching contacts is limited, and setting such a distance is difficult in practice, making this arrangement unsuitable for mass production and hard to realize.
Apart from the above, an idling control system is disclosed in Japanese Patent Publication No. 63-15467 (1988). In the system, the minimum opening degree of the throttle valve is stored in a memory, and a stored value is updated in accordance with an output value from a throttle sensor when the value becomes smaller than the minimum value. At the same time, idling control is performed in dependency on the judgement of the fully closed state of the throttle valve (the idling state) when the output value of the throttle sensor is within a dead zone against the stored value.
The prior art, however, only updates the stored value in dependency on the value output from the throttle sensor when the value of the sensor becomes smaller than the stored value, so that the stored value is updated to be only in the smaller side. Furthermore, the sensor output value becomes higher than the dead zone of the reference value even though the throttle valve is fully closed, when an amount of the sensor output value drifts at the fully closed state of the throttle valve in dependency on such influences as the distortion of a throttle shaft due to connecting a potentiometer as the throttle sensor, strain by negative pressure at the fully closed state, or a temperature change. Therefore, the system of the prior art becomes not to detect the fully closed state of the throttle valve.
Accordingly, we have proposed the following technique, as shown in FIG. 7. A reference value THVISW is set by adding an off-set value .alpha. to a minimum stored value THV.sub.MIN of an output value THV of the throttle sensor. When the idle switch is turned on and when the output value THV of the throttle sensor is over the reference value THV.sub.ISW (=THV.sub.MIN +.alpha.) (lapsed times t4b to t4a), the reference value THV.sub.ISW is corrected in the manner whereby the value THV.sub.ISW is increased by increasing the minimum value THV.sub.MIN in the throttle opening direction by a predetermined value at every predetermined time TIME. As a result, the reference value THV.sub.ISW becomes higher than the output value THV (at t4a), so that the idling state is detected (from t4a to t5).
However, as described above, a driver of the vehicle having the engine with large throttle has much time to lightly depress an accelerator pedal to maintain a very small opening degree of the throttle valve at the constant-speed driving on a straight road. This is because such an engine generates a strong power by a small change of the throttle valve opening.
Accordingly, the output value THV of the throttle sensor becomes THV&gt;THV.sub.ISW when the accelerator pedal is lightly depressed, as shown by lapsed times t5 and t6 in FIG. 7. Then, if the mechanical idling switch maintains 0N state, the minimum stored value THV.sub.MIN is updated by a predetermined value at every predetermined time TIME until reference value THV.sub.ISW becomes over the output value THV of the throttle sensor (to the lapsed time t5a). After that, if the idling switch is turned off further by depressing the pedal (at the lapsed time t6), the minimum stored value THV.sub.MIN is maintained at a present value until the idling state is detected again according to THVISW and THV (a lapsed time t7).
The idling state is determined earlier than a preferable timing during closing the throttle valve (lapsed times t7 and t8) because the reference value THV.sub.ISW set by the minimum stored value THV.sub.MIN is raised to a higher value. As a result, the conventional apparatus has the problems of deteriorating controllability of an air-fuel ratio, an ignition timing at the begining of coasting drive, thereby making it more difficult to drive the vehicle.